DC-DC converters are used to convert an input DC voltage to an output DC voltage. A pulse width modulation (PWM) signal is used to create a DC output in the DC-DC converter. The duty cycle (percentage of time high) of the PWM signal determines the DC output voltage in steady state of the converter
The PWM signal is sent to power switches that send an input voltage into an LC circuit, which includes an inductor and an output capacitor, to produce the output voltage. The output voltage is used to create a feedback signal. The feedback signal is used to adjust the PWM signal so as to keep the output voltage at the desired DC output voltage value.
The maximum load transient response speed in DC-DC converter is limited by the values of the power stage's inductor and capacitor, as well as with the values of input and output voltages. In modern DC-DC converters with a significant step-down ratio, these limitations make heavy-to-light load transient response significantly slower than the response to the opposite transients causing significant voltage overshoot and/or requiring larger output capacitors.
In a light-to-heavy load transient response, the DC output voltage dips and the duty cycle can be increased up to 1 (from its normal relatively low value) to quickly adjust the DC output voltage. On the other hand, in a heavy-to-light transient response, the DC output voltage spikes and the duty cycle can only be dropped to zero (from its normal relatively low value). Since zero is close to the normal relatively low duty cycle value, the system will take longer to respond to the heavy-to-light transient response. To minimize this problem, in most topology-based methods, filter inductance is altered or bridged during transient to result in a higher current slew rate than in steady state.